The invention relates to a method of controlling a d.c. motor of a washing machine in dependence on a parameter which is not exclusively related to motor speed. In such a method the motor is energized from an alternating current supply through a rectifier bridge which comprises two diodes and two thyristors, the thyristors being controlled by a signal which is supplied by an electronic speed regulator. During at least one specific operating cycle of the machine the timer switches on a control device, which supplies a motor de-energize signal to the regulator for stopping the motor when the motor speed drops below a certain value, the control device being actuated by a motor stop signal supplied from a comparator which continually receives both the back-e.m.f. voltage of the motor and a rectified reference voltage from a reference voltage generator.
Such a device for controlling the operation of the motor is particularly useful in a machine which accelerates after a laundry distribution cycle. Such a method of starting up is employed if the weight of the machine is to be reduced or if the spindrying speed is to be increased. It is then necessary to take special care that the mass of laundry is distributed as uniformly as possible along the walls of the drum so as to avoid unbalance. When the mains supply of the machine is interrupted, the motor may either stop because the failure lasts some time, or may slow down when the failure is very brief. In the first case the laundry drops again onto the bottom of the drum; and during speeding up the unbalance will become more severe, depending on the wetness of the laundry. In the second case the loss of speed of the drum may be such that the result will be the same as if it had stopped.
Control of the motor by the control device in the event of a voltage failure is based on the comparison of two voltages: a negative voltage produced by the back-e.m.f. of the motor when the motor has reached its normal operating condition for the relevant cycle and a full-wave rectified reference voltage which is derived from the supply circuit of the regulator and an antihunting device. The reference voltage is adjusted to a value lower than that of the BEMF voltage which corresponds to the drum speed at which the laundry no longer remains at the periphery of the drum.